This invention relates to a unique torsional vibration damper for a system which transfers torque from an engine to a transmission.
Torque transmission systems are widely used in vehicle powertrains. In a typical powertrain, an engine produces torque which is transmitted from an engine crankshaft to a transmission input shaft via the torque transmission system. The transmission transmits torque to a drive shaft coupled to an axle differential, which transmits torque to the vehicle wheels. Because the engine delivers the driving torque for the vehicle by means of various rotatable components, the engine is a source for vibration excitation on the vehicle.
Piston engines have reciprocating pistons in a cylinder, which by definition involves a cyclic process. Thus, the torque delivered by the engine is not constant in magnitude. Instead, the torque consists of a series of pulses which correspond to individual cylinder strokes. The amplitude of the pulses depends on how powerful the engine is and the frequency of the pulses fluctuates with engine speed. As engines become more and more powerful, the fluctuation of engine output speed has increased significantly which in turn has increased the vibration excitation on the vehicle. This is undesirable as it causes problems such as noise, gear rattling, and decreased component life.
Known torque transmission systems comprised of dual flywheels have been used to dampen engine vibrations. These systems are typically located between the engine and the transmission and receive input torque from the engine crankshaft and transmit the torque to the transmission while damping out torsional vibrations. Typically the torque transmission system includes a first flywheel connected to the crankshaft and a second flywheel connected to the transmission input shaft. Each flywheel can move independently relative to the other. Torsional vibration dampers are operably located between the first and second flywheels and resist relative angular movements between the first and second flywheels. Various types of torsional vibration dampers are known in the art, including hydraulic dampers which utilize viscous fluid, elastic dampers such as springs, and frictional dampers. Often a hydraulic damper is used in addition to an elastic damper to adequately reduce torsional vibrations at both high and low engine speeds.
The torsional vibration damper must have high stiffness and high damping at low engine speeds for reducing resonance vibrations, and low stiffness and low damping at high engine speeds so that vibrations can be reduced. The overall goal is to reduce the transmissibility at practical engine speeds. Transmissibility is defined as a measure of the ability of a system to either amplify or suppress an input vibration, equal to the ratio of the response amplitude of the system in steady-state forced vibration to the excitation amplitude; the ratio may be in forces, displacements, velocities, or accelerations. To decrease the transmissibility at practical engine speeds, the resonance frequency must be designed to be within the range below practical engine speeds. This placement of the resonance frequency in relation to engine speed is controlled by stiffness and inertia. However, if the resonance frequency is below practical engine speeds then the frequency of the engine rotating speed will necessarily coincide with the resonance frequency at some point during the starting or stopping of the engine. To avoid a resonance problem at these low engine speeds, the resonance peak or maximum amplitude must be reduced. Damping by means of a torsional damper controls the amplitude of the resonance peak.
Certain deficiencies exist with known torsional vibration dampers. Usually more than one damper is required to accomplish the dual purpose of controlling resonance frequency and reducing the amplitude thereof at variable engine speeds. Typically, elastic dampers such as coil springs are used to provide stiffness at low engine speeds while hydraulic dampers take effect at higher engine speeds. Thus, more parts are required. Packaging difficulty is increased as more dampers are used. Therefore, there is a need for single torsional vibration damper of simple construction which can accomplish these dual purposes.
Also, there are additional deficiencies with prior art hydraulic dampers. Most hydraulic dampers are comprised of various machined components which move in relation to each other. When two machined surfaces contact each other for the purpose of preventing fluid flow, leakage can occur due to manufacturing inaccuracies. Additionally, these hydraulic dampers have complex sealing systems to prevent fluid loss to other areas of the torque transmission system. Thus, there is a need for a simple, self contained hydraulic damper which eliminates leakage resulting from manufacturing inaccuracies and which simplifies the sealing system used to prevent fluid loss to other areas of the torque transmission system.